The present invention relates to a switching power supply circuit.
In recent years, as electronic apparatuses have become compact in size, secondary batteries and dry batteries have been used generally as power supply sources for such electronic apparatuses. Since the voltage of a secondary battery or a dry battery varies depending on its discharge time (operating time), it is usually necessary to stabilize the voltage using a power supply circuit built in an electronic apparatus. Furthermore, for the purpose of attaining long time operation, in recent years there is an increasing strong demand for operation on secondary batteries or dry batteries at voltages lower than ever before.
A switching power supply circuit in accordance with prior art No. 1 satisfying this kind of demand has been disclosed in Japanese Patent No. 3138218. The switching power supply circuit in accordance with this prior art No. 1 will be described referring to FIG. 3.
The input power supply 102 shown in FIG. 3 consists of secondary batteries or dry batteries. In FIG. 3, components other than the input power supply 102 constitute the switching power supply circuit in accordance with the prior art No. 1. The connections of the respective components of the switching power supply circuit in accordance with the prior art No. 1 will be described below.
The input terminal 111 of the switching power supply circuit in accordance with the prior art no. 1 is connected to one terminal of the input power supply 102, batteries, and an input voltage Vin is applied thereto. The other terminal of the input power supply 102 is connected to the grounding point of the circuit. A choke coil 107 and an NPN transistor 311 (switching NPN power transistor) are connected in series between the input terminal 111 and the grounding point.
The anode of a diode 312 is connected to the connection point J31 between the choke coil 107 and the NPN transistor 311. The cathode of the diode 312 is connected to the output terminal 112 of the switching power supply circuit.
One terminal of an input smoothing capacitor 103 is connected to the connection point J32 between the input terminal 111 and the choke coil 107, and the other terminal thereof is connected to the grounding point. One terminal of an output capacitor 106 is connected to the connection point J33 between the diode 312 and the output terminal 112, and the other terminal thereof is connected to the grounding point.
The anode of a diode 314 is connected to the connection point J34 between the cathode of the diode 312 and the output terminal 112. The cathode of the diode 314 is connected to one terminal of a resistor 316, and the other terminal of the resistor 316 is connected to the grounding point.
The emitter of a PNP transistor 315 is connected to the input terminal 111, its base is connected to the connection point J35 between the diode 314 and the resistor 316, and its collector is connected to the power supply terminal Vcc of a power supply control circuit 101. The PNP transistor 315 serves as a path through which power is supplied from the input terminal 111 to the power supply terminal Vcc of the power supply control circuit 101.
The anode of the diode 313 is connected to the output terminal 112, and its cathode is connected to the power supply terminal Vcc of the power supply control circuit 101. The diode 313 serves as a path through which power is supplied from the output terminal 112 to the power supply terminal Vcc of the power supply control circuit 101.
Both the cathode of the diode 313 and the collector of the PNP transistor 315 are connected to the power supply terminal Vcc of the power supply control circuit 101, and power is supplied. The base of the NPN transistor 311 is connected to the control terminal VB of the power supply control circuit 101, and the switching operation of the NPN transistor 311 is controlled.
The output terminal 112 outputs an output voltage Vout. This output voltage Vout is fed back to the negative feedback terminal (terminal FB) of the power supply control circuit 101.
The operation of the switching power supply circuit in accordance with the prior art No. 1 configured as described above will be described below. First, the operation at the start of voltage step-up operation (at the start of switching operation) will be described. When the input voltage Vin is input from the input power supply 102 to the input terminal 111, the PNP transistor 315 is turned ON, and the input voltage Vin is applied to the power supply terminal Vcc of the power supply control circuit 101 via the PNP transistor 315.
When the PNP transistor 315 is in the ON state, the voltage between the collector and the emitter has a very small value, whereby a voltage nearly equal to the input voltage Vin is applied to the power supply terminal Vcc. For example, when it is assumed that the operation start lower-limit voltage V0 of the power supply control circuit 101 is 3.0 V and that the saturation voltage Vce between the collector and the emitter of the PNP transistor 315 is 50 mV, the power supply control circuit 101 starts operation if the input voltage Vin output from the input power supply 102 is 3.05 V.
When the power supply control circuit 101 starts operation, the NPN transistor 311 is driven by a drive signal from the control terminal VB of the power supply control circuit 101 and carries out switching operation. When the NPN transistor 311 is turned ON, the NPN transistor 311 feeds a supply current to the choke coil 107, and energy is stored therein. The diode 312 for rectification rectifies the counter electromotive force generated at the connection point J31 on the basis of the stored energy when the current is cut off. The current generated by the counter electromotive force is applied to the output capacitor 106 via the rectifying diode 312. The output capacitor 106 is charged with this, whereby the output voltage Vout at the output terminal 112 is stepped up.
The output voltage Vout is fed back to the terminal FB of the power supply control circuit 101. The power supply control circuit 101 controls the switching operation of the NPN transistor 311 on the basis of the output voltage Vout input to the terminal FB so that the output voltage Vout becomes a constant voltage. In this way, the output voltage Vout is negatively fed back and becomes the constant voltage.
In the step-up switching power supply circuit shown in FIG. 3, the output voltage Vout has a voltage value higher than the input voltage Vin. When the potential of the output terminal 112 becomes higher than the potential of the input terminal 111, the diode 314 conducts and raises the potential of the base (the potential of the connection point J35) of the PNP transistor 315, thereby turning OFF the PNP transistor 315. In this way, when the output voltage Vout becomes higher than the input voltage Vin, the PNP transistor 315 is turned OFF, and the output voltage Vout is supplied to the power supply terminal Vcc of the power supply control circuit 101 via the forward diode voltage of the diode 313.
Next, the operation during the stop (standby) of the voltage step-up operation will be described. During the stop (standby) of the voltage step-up operation, the power supply control circuit 101 outputs a drive signal from its control terminal VB. The switching operation of the NPN transistor 311 is stopped by the drive signal, and its OFF state is maintained.
The output voltage Vout lowers owing to discharge due to the consumption current of the power supply control circuit 101. When a sufficient time has passed after the stop of the voltage step-up operation, the output voltage Vout has a value represented by the following expression (1). The Vf312 in the expression is the forward diode voltage of the rectifying diode 312.Vout=Vin−Vf312  (1)
If the three devices, the diode 314, the PNP transistor 315 and the resistor 316, are not present in this state, the voltage V101 applied to the power supply terminal Vcc of the power supply control circuit 101 has a value represented by the following expression (2). The Vf313 in the expression is the forward diode voltage of the diode 313.V101=Vin−Vf312−Vf313  (2)
When it is assumed that the operation start lower-limit voltage of the power supply control circuit 101 is V0, the operation start lower-limit voltage Vs viewed from the input power supply 102 is represented by the following expression (3).Vs=V0+Vf312+Vf313  (3)
The operation start lower-limit voltage Vs viewed from the input power supply 102 becomes higher than the actual operation start lower-limit voltage V0 of the power supply control circuit 101 by approximately 1.2 to 1.4 V (the voltage values of the forward diode voltages Vf312 and Vf313 of the diode 312 and the diode 313).
Since the switching power supply circuit in accordance with the prior art No. 1 has the diode 314, the PNP transistor 315 and the resistor 316, the operation start lower-limit voltage Vs is made lower. Hence, the operation of the power supply control circuit is made possible at the lower operation start lower-limit voltage Vs. This will be described below in detail.
During the stop of the voltage step-up operation, the output voltage Vout is lower than the input voltage Vin as described in the expression (1). A current flows from the emitter of the PNP transistor 315 to its base and the resistor 316. The base voltage Vb315 of the PNP transistor 315 is represented by the following expression (4). The Vbe315 in the expression (4) is the base-emitter voltage of the PNP transistor 315.Vb315=Vin−Vbe315  (4)
Since the base voltage (the voltage at the connection point J35) of the PNP transistor 315, represented by the expression (4), is higher than the output voltage Vout represented by the expression (1), the diode 314 is in the OFF state. The PNP transistor 315 is fully turned ON and saturated. Since the PNP transistor 315 is saturated, the voltage V101 applied to the power supply terminal Vcc of the power supply control circuit 101 is represented by the following expression (5). The Vce315 in the expression (5) is the saturation voltage between the collector and emitter of the PNP transistor 315.V101=Vin−Vce315  (5)
The saturation voltage Vce315 between the collector and emitter of the PNP transistor 315 is far lower than the forward diode voltages Vf312 and Vf313 of the diode 312 and the diode 313. The voltage V101 in the expression (5) is higher than the voltage V101 in the expression (2) by approximately 2Vf=1.2 to 1.4 V.
The operation start lower-limit voltage Vs viewed from the input power supply 102 is represented by the following expression (6) when it is assumed that the operation start lower-limit voltage of the power supply control circuit 101 is V0.Vs=V0+Vce315  (6)
Since the saturation voltage Vce315 is low, the operation start lower-limit voltage Vs viewed from the input power supply 102 has a value almost equal to or slightly higher than the actual operation start lower-limit voltage V0 of the power supply control circuit 101. The difference between the voltages in the expressions (3) and (6) is very large in the case of electronic apparatuses operating on secondary batteries or dry batteries.
As described above, in the switching power supply circuit in accordance with the prior art No. 1, the operation start lower-limit voltage can be lowered, and the input voltage Vin output from the input power supply 102 can have a low value. For example, in the case that the operation start lower-limit voltage V0 of the power supply control circuit 101 is 3 V, the switching power supply circuit in accordance with the prior art No. 1 can be started by using two dry batteries connected in series and having an output voltage of 3.3 V as an input power supply.
In the case that dry batteries are used as the input power supply 102, the reverse current flowing from the output capacitor 106 to the input power supply 102 degrades the dry batteries and shortens their service lives. It is therefore necessary to avoid the reverse current to the utmost.
When the input voltage Vin is higher than the output voltage Vout, the switching power supply circuit in accordance with the prior art No. 1 drives the PNP transistor 315 that forms a power supply path to the power supply control circuit 101. When the output voltage Vout is higher than the input voltage Vin, the switching power supply circuit in accordance with the prior art No. 1 drives the diode 313 that forms a power supply path to the power supply control circuit 101. Hence, in the switching power supply circuit in accordance with the prior art No. 1, the PNP transistor 315 and the diode 313 turn ON and OFF complementarily, whereby reverse current flow from the output capacitor 106 to the input power supply 102 is not generated.
In recent years, there is an increasing trend toward reducing the power consumption of electronic apparatuses, such as portable telephones, DSCs (Digital Still Cameras). The power conversion efficiency of the switching power supply circuit has thus become a very important factor.
The switching power supply circuit in accordance with the prior art No. 1 disclosed in Japanese Patent No. 3138218 has the consumption of the base current (switching device driving current) during the switching of the NPN transistor 311 and a power loss (power loss during rectification) due to the forward diode voltage Vf312 of the rectifying diode 312. Hence, the switching power supply circuit of the prior art is disadvantageous in power conversion efficiency.
For this reason, at present, a switching power supply circuit in accordance with prior art No. 2 using a MOSFET (Metal Oxide Semiconductor Field-Effect Transistor) switching device and MOSFET synchronized rectification instead of the NPN transistor 311 and the rectifying diode 312 has become mainstream.
The switching power supply circuit in accordance with the prior art No. 2 will be described referring to FIG. 4. FIG. 4 is a circuit diagram showing the configuration of the switching power supply circuit in accordance with the prior art No. 2. The switching power supply circuit in accordance with the prior art No. 2 has an input terminal 111 connected to one terminal of an input power supply 102 consisting of batteries; an input smoothing capacitor 103, one terminal of which is connected between the input terminal 111 and a choke coil 107 and the other terminal of which is connected to the grounding point of the circuit; the choke coil 107 and an N-channel MOSFET 104 connected in series between the input terminal 111 and the grounding point; a P-channel MOSFET 105, the drain of which is connected to the connection point J41 between the choke coil 107 and the N-channel MOSFET 104; an output terminal 112 connected to the source of the P-channel MOSFET 105; a power supply control circuit 101, the power supply terminal Vcc of which is connected to the output terminal 112, the control terminal VG1 of which is connected to the gate of the N-channel MOSFET 104 and the control terminal VG2 of which is connected to the gate of the P-channel MOSFET 105; and an output capacitor 106, one terminal of which is connected to the connection point J43 between the P-channel MOSFET 105 and the output terminal 112, and the other terminal of which is connected to the grounding point. In FIG. 4, the same components as those shown in FIG. 3 are designated by the same numerals.
The switching power supply circuit in accordance with the prior art No. 2 has a power loss (power loss during rectification) due to a voltage drop determined by [ON resistance×current] at the P-channel MOSFET 105 for synchronized rectification.
The voltage drop at the P-channel MOSFET 105 is far lower than the forward diode voltage of the rectifying diode 312 shown in FIG. 3. The power loss due to the rectifying operation of the P-channel MOSFET 105 is less than the power loss at the rectifying diode 312. Hence, the switching power supply circuit in accordance with the prior art No. 2 is improved in power conversion efficiency in comparison with the switching power supply circuit in accordance with the prior art No. 1.
Next, the operation of the switching power supply circuit in accordance with the prior art No. 2 at the start of voltage step-up operation (at the start of switching operation) will be described below. When an input voltage Vin is applied from the input power supply 102 to the input terminal 111, a current flows through the parasitic diode of the P-channel MOSFET 105. The output voltage Vout obtained at this time is represented by the following expression. In the following expression, Vd is the forward diode voltage of the parasitic diode of the P-channel MOSFET 105 and is approximately 0.7 V.Vout=Vin−Vd 
When the output voltage Vout reaches the operation start lower-limit voltage V0 of the power supply control circuit 101 (V0=Vin−Vd), the power supply control circuit 101 generates a drive signal, whereby the N-channel MOSFET 104 and the P-channel MOSFET 105 carry out switching operation. Hence, the output voltage Vout is stepped up to a predetermined voltage. Since the parasitic diode of the P-channel MOSFET 105 is reverse biased by the output voltage Vout having been stepped up, it is turned OFF when the switching power supply circuit starts voltage step-up operation.
The switching power supply circuit in accordance with the prior art No. 2 does not operate when the input voltage Vin is not higher than the operation start lower-limit voltage V0 of the power supply control circuit 101 by the forward diode voltage Vd (approximately 0.7 V) of the parasitic diode (Vin=V0+Vd) or more. For example, when the operation start lower-limit voltage V0 of the power supply control circuit 101 is 3 V, the switching power supply circuit in accordance with the prior art No. 2 does not started using two dry batteries connected in series and having an output voltage of 3.3 V.
In the switching power supply circuit in accordance with the prior art No. 1, the operation start lower-limit voltage can be lowered, but high power conversion efficiency cannot be attained. In the switching power supply circuit in accordance with the prior art No. 2, high power conversion efficiency can be attained, but the operation start lower-limit voltage cannot be lowered.
In consideration of this, it is conceivable that the circuit devices (the diode 313, diode 314, PNP transistor 315 and resistor 316) in accordance with the prior art No. 1 capable of lowering the operation start lower-limit voltage are installed in the switching power supply circuit in accordance with the prior art No. 2 capable of attaining high power conversion efficiency. However, if the circuit devices (the diode 313, diode 314, PNP transistor 315 and resistor 316) in accordance with the prior art No. 1 are installed in the switching power supply circuit in accordance with the prior art No. 2, the circuit thus obtained does not operate properly as described below.
It is assumed that the diode 313 is inserted in the path from the output terminal 112 to the power supply terminal Vcc of the power supply control circuit 101 in the switching power supply circuit in accordance with the prior art No. 2. When the output voltage Vout having been stepped up becomes higher than the input voltage Vin, the voltage supplied to the power supply terminal Vcc of the power supply control circuit 101 becomes [the output voltage Vout−the voltage drop Vf313 of the diode 313]. The High-level gate voltage of the synchronized rectification MOSFET 105, output from the power supply control circuit 101, becomes lower than the source voltage of the MOSFET 105 by Vf313 (approximately 0.7 V), and the P-channel MOSFET 105 is not turned OFF. Hence, the power supply control circuit 101 cannot properly drive the P-channel MOSFET 105, and the P-channel MOSFET 105 remains ON.
If only the diode 314, PNP transistor 315 and resistor 316 among the circuit devices of the prior art No. 1 are installed in the switching power supply circuit in accordance with the prior art No. 2 (the diode 313 is short-circuited), the circuit thus obtained does not operate properly as described below.
When the output voltage Vout having been stepped up becomes higher than the input voltage Vin, the collector voltage of the PNP transistor 315 becomes Vout, and the base voltage of the PNP transistor 315 becomes [the output voltage Vout−the voltage drop Vf314 of the diode 314]. Hence, the base-collector voltage Vbc of the PNP transistor 315 becomes approximately 0.7 V, and the PNP transistor 315 becomes ON at all times. An unnecessary current flows reversely from the collector to the emitter of the PNP transistor 315 and to the input power supply 102. In particular, in the case that the input power supply 102 consists of primary batteries, such as dry batteries, the phenomenon of the reverse flow of the current to the primary batteries 102 will abruptly shorten the service lives of the primary batteries 102.
Consequently, it is impossible to attain both high power conversion efficiency and low operation start lower-limit voltage by installing the circuit devices (the diode 313, diode 314, PNP transistor 315 and resistor 316) in accordance with the prior art No. 1 capable of lowering the operation start lower-limit voltage in the switching power supply circuit in accordance with the prior art No. 2 capable of attaining high power conversion efficiency. Conventionally, switching power supply circuits capable of attaining both high power conversion efficiency and low operation start lower-limit voltage have not been present.